Battery-powered devices (e.g., consumer electronic devices, electric and hybrid automobiles, etc.) are charged from a power source (e.g., AC power outlet) through a charging device. The charging device couples the battery to the power source through an adaptor. The cord extending between the power source and the battery-powered device can take up space. In situations where multiple devices require charging, each with their own charger and cord, the charging area can become cramped and inconvenient.
Approaches are being developed that use over-the-air or wireless power transmission between a transmitter and a receiver coupled to the electronic device. Wireless power transmission using inductive coils is one method considered as an un-tethered method for transferring power wirelessly through a coupled electromagnetic field. In wireless power transmission, power is transferred by transmitting an electromagnetic field through a transmit coil. On the receiver side, a receiver coil may couple with the transmit coil through the electromagnetic field, thus, receiving the transmitted power wirelessly. The distance between the transmitter and receiver coils, at which efficient power transfer can take place, is a function of the transmitted energy and the required efficiency. The coupling coefficient (k) is a function of the distance between the coils, the coil sizes, and materials. The power conversion efficiency (e.g., coupling factor, coupling quality) may be significantly improved if the coils are sized and operated at such a frequency that they are physically within the so-called “near-field zone” of each other.
Wireless power systems are generally intended to operate in a frequency range substantially near (e.g., exactly at) the peak resonance of the resonant tanks of the wireless power devices. Different wireless power devices may be configured differently, which may cause the devices to be incompatible. For example, a wireless power transmitter may be designed to operate according to a peak resonance that is substantially different than the peak resonance of a wireless power receiver. As a result, if one were to attempt to establish wireless power transfer between two dissimilar devices, wireless power transfer parameters such as the system efficiency and the transient response of the wireless power receiver may suffer.